Third Cousin.
The terms 'great cousin' and 'grand cousin' are not ones used in genealogy. Your family may use the term, but it is not in common usage and has no standard meaning.
All of your cousin's descendents are related to you. Your first cousin's grand chid is your first cousin, twice removed.
Fred.
2nd cousin or 2nd cousin in law.
Your second cousin is the grand child of the brother or sister of one of your grand parents.
Since your niece is your daughter's first cousin, your grand niece (the daughter of your niece) is your daughter's first cousin, once removed.
The cousin of your cousin is related to you if you share a common ancestor, such as if you are all descended from the same great grand father. If the cousin of your sousin is related to your cousin through a person to whom YOU are not related, then you are not related to the cousin of your cousin.
Your dad's first cousins child is your second cousin. Your dad's first cousin's grand child is your second cousin, once removed.
Mon grand cousin est gras et mon petit cousin est maigre
Your great aunt's son is your parent's first cousin and your first cousin, once removed.
He would be your grand-nephew.